1. Field of the Invention
The present invention relates to a sheet aligning method in a sheet post-processing apparatus provided at a post stage of an image forming apparatus and a sheet post-processing apparatus including an aligning device.
2. Description of the Related Art
As it is well known, in recent years, an image forming apparatus having plural functions is spread. A sheet post-processing apparatus including a stapler for stapling sheets and a saddle unit for performing sheet center folding and center binding is set adjacent to such an image forming apparatus.
U.S. Pat. No. 7,172,187 discloses a sheet post-processing apparatus having a waiting tray for moving a pair of tray parts in a sheet width direction to drop sheets. In the waiting tray, every time a sheet is conveyed to the waiting tray, waiting tray rollers align the sheet to a rear end of the waiting tray. The speed of supply rollers provided immediately before the waiting tray is adjusted according to a sheet size to fix a sheet standby position in the waiting tray with respect to a sheet feeding direction. Depending on the length of a conveyed sheet, when the sheet is conveyed from a conveying path to the waiting tray, a portion in the center of the waiting tray not supported by the pair of tray parts bends downward in a U shape and a alignment disorder phenomenon of a trailing end of the sheet occasionally occurs on the waiting tray. This leads to an alignment failure phenomenon in a processing tray.
In order to vertically align, on the processing tray, sheets dropped onto the processing tray through the waiting tray, plural aligning member may be provided in such a processing tray to come into contact with an uppermost surface and a lowermost surface of the sheets (a bundle). For example, in JP-A-63-180673, a waiting tray includes a fixed section and a moving section and sheets are discharged from the waiting tray to a processing tray by the moving section. When the sheets are discharged to the processing tray, in the processing tray, an upper surface and a lower surface of the sheets are aligned by an aligning belt and a segmental roller, respectively.
Aligning member provided to come into contact with a lower surface of a bundle of sheets stacked on the processing tray acts on only a first (bottom) sheet. However, there is a disadvantage that, when the aligning member is actuated consecutively, a trace is left on the surface of sheets or the surface of a print is stained. Moreover, when it is attempted to increase the speed of post-processing, because of the influence of collision with a dropped sheet, vibration of an apparatus, an air flow in the apparatus, and the like, upper sheets in stacked sheets move or a bottom sheet gradually moves when the sheets are consecutively discharged and stacked.
According to a request for an increase in speed of sheet post-processing, it is desired to improve and stabilize alignment accuracy of an aligning device. Therefore, it is an object of the present invention to provide a sheet post-processing apparatus including an aligning device that aligns sheets conveyed from an image forming apparatus on a waiting tray and a processing tray without being disorderly stacked.